Traditional audio power amplifiers use positive and negative power transistors, connected to positive and negative voltage rails, operating in a variable-resistance mode to pass the desired positive and negative currents to the speaker. Well-known techniques are used to minimize amplification errors. However, efficiency is inherently limited by resistive losses and is particularly low at ordinary output levels, where only a fraction of the power supply voltage is passed to the load. Efficiency can be significantly improved by using intermediate-rail voltages with steering circuitry to reduce the average voltage drop across the power transistors, but it adds complexity and introduces new sources of audio distortion. As a result, it becomes impractical to mount a very high power amplifier on or within a speaker because the heat sink will be too large and fan cooling is undesirable due to the noise and dust buildup.
A newer technique uses “high frequency switching” to replace wasteful linear power control. In this technique, transistors are used as ON-OFF switches rather than as variable resistors, and their duty cycle determines the percentage of the power supply voltage that reaches the load. Such switching can be highly efficient, since there is no current through the device when “OFF” and very low voltage across the device when “ON”, thus avoiding the simultaneous dissipation of high voltages and currents that characterize the linear solution.
Known requirements include very rapid switching to reduce the time spent in the “linear” region, at a frequency well above the highest audio frequency, to permit an averaging filter to reconstruct a smooth and continuous audio waveform. Further requirements include an accurate “modulator” that converts the audio signal into a pulse width modulated (PWM) signal whose ON/OFF ratio accurately represents the instantaneous amplitude of the audio signal, followed by high-frequency power transistors with sufficient voltage and current capacity to deliver the desired power, followed by an inductor-capacitor output filter that integrates the power pulses, passes the audio-band signals, and attenuates the switching frequencies. A further requirement is a DC power supply with predictable and reasonably well-regulated voltages, to maintain the switching devices within their ratings.
The existing Class-D art suffers from:                a) Poor Linearity        b) Poor Power Supply Rejection        c) Poor Frequency Response        d) ON-OFF Noises Caused by Uncontrolled Onset of Switching Activity        e) Overload and Overheating        f) Excessive Switching Losses        g) Excessive Switching Noise        h) Excessive Complexity and CostThe causes of these deficiencies are briefly discussed: (a) Because the transfer function is not uniform over the entire range of modulation, various errors occur after the modulator which result in harmonic distortion (THD) of 1% or more, while high quality amplifiers need less than 0.1% THD. (b) In a system using standard “open-loop” PWM, the PWM ratio causes a fixed percentage of the power supply voltage to reach the load, and therefore any power supply fluctuation appears at the output. This requires either an expensive fully-regulated power supply, or results in hum and amplitude changes in the output signal. The existing “Error correcting” modulators have other disadvantages, as will be noted here. (c) Even in an otherwise perfect system, the L-C output filter has a frequency response and a Q that depend on the output load impedance. Different speakers, loadings, and other conditions make this loading highly variable, causing unpredictable high-frequency response. (d) If switching is not started and stopped in a particular manner, transients are generated, causing “pops” in the speaker. (e) Although theoretically lossless, actual Class-D schemes suffer from finite switching times and resistive losses, thereby generating a certain amount of heat. These losses can be minimized with a slight “dead time” between positive and negative switching transitions, but such dead time is another source of poor linearity. The losses also depend on the magnitude of current flow, which must therefore be limited to some safe level. Known current limiting schemes are complicated by properties of typical switching devices (FETs) whose losses increase with rising temperature. Setting protection limits that are safe at maximum temperatures may unduly limit performance at normal operating temperatures. Furthermore, external current-monitoring schemes may fail to detect excessive currents due to abnormal internal conditions. (f) Since switching losses are proportional to switching frequency, it is desirable to switch at the lowest possible frequency, but audio performance will suffer due to complications in applying overall negative feedback. Existing schemes for internal error correction cause the switching frequency to vary with depth of modulation, thereby requiring the average frequency to be increased in order to maintain the required separation between audio and switching frequencies. Existing open-loop schemes have a fixed frequency but poor linearity and no internal error correction, and therefore also require a high operating frequency to allow the use of external negative feedback. Existing Class B-D schemes require a second output inductor, and still require accurate control of timing signals to prevent long-term thermal losses. (g) A fixed-frequency scheme can use highly selective “notch filters” to attenuate switching frequencies, with minimal effect in the audio passband. Schemes with a variable switching frequency cannot advantageously use such “traps”. (h) In general, these problems complexity and cost have previously been reduced by using higher switching frequencies, premium components, extremely tight tolerances, elaborate circuitry, hand-tuning of each unit, fully regulated power supply voltages, and other costly “brute-force” solutions.        